1. Field of the invention
The present invention relates to a method of driving a wire dot print head which drives print wires to make dot impressions on a printing medium.
2. Description of the Prior Art
Wire-dot printers include an actuator for driving a print wire. The actuator is supplied with an electric power from a power source via a switching element. To drive a print wire, a printing pulse is applied for a predetermined duration to the associated switching element to render it conductive.
Heretofore, it has been proposed to apply a braking pulse to the switching element to render it conductive following the application of the printing pulse thereto. The braking pulse is applied to the switching element for a duration shorter than the duration of the printing pulse so that printing is not carried out in response to the braking pulse. While a high speed printing can be achieved if the braking pulse is not inserted after the printing pulse, the braking pulse is needed for controlling or adjusting the printing speed in order to weaken a stress of a stopper resulting from the impingement of the print wire thereon and to thus suppress secondary bound of the printing wire back from the stopper. Further, the braking pulse needs to be inserted when a special mode of printing such as double speed printing or odd- and even-number dot separate printing is carried out. For such reasons, the braking pulse is inserted for each dot printing.
FIGS. 1 and 2 show curves indicating displacement of a print wire and timing charts of the printing and braking pulses according to conventional driving methods. FIG. 1 shows the case when continuous-shot drive is performed to print a series of dots and FIG. 2 shows the case when single-shot drive is performed to print a single dot in a discontinuous manner. As can be appreciated from FIG. 1, the duration of the printing pulse A and the application timing of the braking pulse B with respect to the associated printing pulse A are the same in the successive print cycles T. In the case of single-shot drive shown in FIG. 2, the duration of the print pulse A is set to be equal to that of the leading printing pulse in the continuous-shot drive.
FIGS. 3 and 4 show another conventional driving method wherein in the case of continuous-shot drive, a braking pulse is applied only after the application of the trailing printing pulse.
As a result of experiments, the present inventor has found that even if the secondary bound of a print wire back from a stopper could be effectively prevented in the single-shot drive by the application of the braking pulse, the same approach is not applicable to the case when continuous-shot drive is carried out. Specifically, the secondary bound of the print wire cannot be prevented from occurring after the impression of trailing dot even if the similar control as in the single-shot drive is adopted for the continuous-.shot drive, as can be appreciated from the waveforms Ma in FIGS. 1 and 3. If the secondary bound of the print wire occurs, the density of printed image become uneven and print ghost appears.